Chip-Level Channel Equalization in WCDMA Downlink
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Chip-Level Channel Equalization in WCDMA Downlink Kari Hooli Centre for Wireless Communications, P.O. Box 4500 FIN-90014, University of Oulu, Finland Email: [email protected]
Markku Juntti Centre for Wireless Communications, P.O. Box 4500 FIN-90014, University of Oulu, Finland Email: [email protected]
Markku J. Heikkila¨ Nokia Mobile Phones, P.O. Box 50, 90571 Oulu, Finland Email: [email protected]
Petri Komulainen Nokia Mobile Phones, P.O. Box 50, 90571 Oulu, Finland Email: [email protected]
Matti Latva-aho Centre for Wireless Communications, P.O. Box 4500 FIN-90014, University of Oulu, Finland Email: [email protected]
Jorma Lilleberg Nokia Mobile Phones, P.O. Box 50, 90571 Oulu, Finland Email: [email protected] Received 14 August 2001 and in revised form 7 March 2002 The most important third generation (3G) cellular communications standard is based on wideband CDMA (WCDMA). Receivers based on TDMA style channel equalization at the chip level have been proposed for a WCDMA downlink employing long spreading sequences to ensure adequate performance even with a high number of active users. These receivers equalize the channel prior to despreading, thus restoring the orthogonality of users and resulting in multiple-access interference (MAI) suppression. In this paper, an overview of chip-level channel equalizers is delivered with special attention to adaptation methods suitable for the WCDMA downlink. Numerical examples on the equalizers’ performance are given in Rayleigh fading frequency-selective channels. Keywords and phrases: WCDMA, multiple-access interference, channel equalization.
1.
INTRODUCTION
The air interface of universal terrestrial radio access (UTRA), the most important third generation (3G) cellular mobile communications standard, is based on wideband codedivision multiple-access (WCDMA). In 3G, cellular networks downlink capacity is expected to be more crucial than uplink capacity due to asymmetric capacity requirements, that is, the downlink should offer higher capacity than the uplink [1]. Therefore, the use of efficient downlink receivers is important. In order to avoid performance degradation, near-far resistant (or multiuser) receivers [2]
can be used. Several suboptimal receivers feasible for practical implementations have been proposed, including linear minimum mean-squared error (LMMSE) receivers [3, 4]. The adaptive versions of LMMSE receivers are the most promising for single-user terminals. However, the adaptive symbol-level LMMSE receivers rely on cyclostationarity of multiple access interference (MAI), and thus require periodic spreading sequences with a very short period. Hence, they cannot be applied to the frequency division duplex (FDD) mode of the WCDMA downlink, which uses spreading sequences with a one radio frame (10 ms) period.
758 In a synchronously transmitted downlink employing orthogonal spreading codes, MAI is mainly caused by multipath propagation (neighboring cells form another source of MAI). Due to the nonzero cross-correlations
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